The present invention particularly relates to a handover operation in a Time Division Multiple Access (TDMA) digital cellular radio communication system comprising a plurality of Base Transceiver Stations (BTS), and a number of Mobile Stations (MS).
Analog mobile phone systems have long been available, the basic notion thereof being that a network is composed of a plurality of mutually defined cells, each provided with a Base Station in which the transmission power covers the range of said cell, but scarcely extending to the range of an adjacent cell. The system also includes Mobile Services Switching Centers, with each of which being connected a great number of Base Stations. In addition, the system includes a plurality of mobile phones, usually called Mobile Stations (MS), so that a connection can be formed between two mobile phones or between a subscriber connected to the Public service telephone network (PSTN) and a mobile phone. Said networks are frequency divided, whereby one connection always engages two frequencies. The completely digital systems based on TDMA represent the next step of development in mobile phone systems in which the same frequency is used in several connections but at different periods of time. Of such systems may be mentioned the GSM currently being adopted into use in Europe and the digital cellular system to be used in the USA.
In all cellular networks measures have to be taken in one way or another to enable a mobile phone to move from one channel to another during a call without breaking the connection. A channel change, i.e. handover, can be intracellular, whereby the movement is made from one channel or time interval to another while remaining within one Base Transceiver Station, or intercellular, so that the connection changes to another Base Transceiver Station, this being subjected to either the same Base Station Controller or the Base Transceiver Station is subjected to a second Base Station Controller. Potential cases are shown in FIG. 1 illustrating the principle block diagram of a typical digital system. As shown therein, several Base Station Controllers BSC have been connected to a Mobile services switching centre MSC, each controlling a number of Base Transceiver Stations BTS. Each BSC together with the Base Stations connected thereto makes up a Base Station System BSS in which the control of a cellular network has been concentrated in the BSC.
A connection with other networks, such as the PSTN, is carried out through MSC. The handover can be made intracellularly, whereby a Mobile Station MS is in connection with one and the same Base Station (e.g. BTS 1) but it changes the frequency or the time interval of the trafficking channel if the level of interference on a certain channel has become too great. The handover decision is usually made in the BSC. Said intracellular handover is indicated by a. When changing a connection from one Base Transceiver Station (e.g. BTS 1) to another Base Transceiver Station (e.g. BTS 2) subjected to the same Base Station controller, the handover decision is made also in the BSC 1. Said move is illustrated by arrow b. A connection may also be moved as shown by arrow c from a Base Transceiver Station (e.g. BTS 3) to a Base Transceiver Station (e.g. BTS 21) subjected to another Base Station Controller (e.g. BSC 2). Now, the control of the move is carried out by the Mobile Services Switching Center (MSC).
A great number of measures are required in carrying out a perfect handover in prior art digital time divided radio phone systems. A Mobile Station MS must measure, not only the signal strength of the Base Station to which it is connected but also those of the adjacent Base Stations and to transmit measuring results to the Base Transceiver Station BTS in use at that moment. The Base Station transmits the data to an MSC which makes the decision on which Base Transceiver Station the call is to be transferred. In addition to measurements concerning the signal strengths of the adjacent Base Stations, the mobile station is required from time to time to discharge a given burst transmitted by the adjacent Base Stations, on the basis of which the telephone calculates the timing difference between the Base Station to which is connected and the timings of the adjacent Base Stations using the parameters derived from said Base Stations. Similarly, the frame number difference between the Base Stations in use own and the adjacent Base Stations has to be calculated.
Also the timing of starting a transmission detected by the Base Station has to be taken into account so that the synchronization with the Base Station can be maintained while the Mobile Station is moving at a long distance from the Base Station.
As a result of the measurements carried out by the Mobile Station and the Base Station, the mobile phone network determines the handover moment and to which new Base Station the connection is going to be moved. All of said operations related to the handover between the MSC, the BTSs and the Mobile Station MS require a great number of signalling and signal processing operations in the network. Therefore, in digital networks known in the art the handover operation is bound to be rather complicated and heavy.
In TDMA systems of prior art, the multiple access propagation of a signal in a radio channel is seen in the form of mutual influence between the detected bits, and the changing timing causes variations in the reception. The receivers are required to be able to correct said time-related variations produced in a radio channel over the length of several symbols, e.g. in the GSM system reservations have been made to correct delay dispersions up to 16 microseconds.
A procedure is disclosed for enabling a handover to be carried out simply in a digital, TDMA cellular system.
The handover according to the present invention can be applied to a TDMA cellular network as described above.